A yeast-based bioassay for the determination of functional and non-functional estrogen receptors.

The response to endocrine therapy of breast cancer is not entirely predictable from hormone receptor status alone since some point mutated or splicing variants of the estrogen receptor (ER) show altered biological activities. In order to characterize the activities of all forms of ER in a heterogeneous breast tumor, a functional assay in Saccharomyces cerevisiae was developed. Total RNA isolated from breast cancer cells and one breast cancer specimen was reverse transcribed and the ER cDNA was amplified by PCR. The products were then cloned into an expression vector by in vivo homologous recombination in yeast. The yeast strain carries a reporter gene ( ADE2 ) coupled to an estrogen response element. Activation of the reporter by ER yielded white colonies whereas lack of ER activity produced red colonies. This permitted the testing for functionality of individual ER molecules and subsequent analysis by rescuing of the ER expression plasmids and complete DNA sequencing. This simple visual test allows discrimination between wild-type ER, constitutively active ER and inactive ER.

Increased expression of estrogen-receptor exon-5-deletion variant in relapse tissues of human breast cancer.

A substantial percentage (30-70%) of human breast carcinomas that initially respond to endocrine therapy acquire resistance during the treatment. Many patients with tumor progression despite treatment with anti-estrogen tamoxifen show continued expression of estrogen receptors (ER) and/or progesterone receptors (PgR) in the relapse tissue. This indicates that, in these tumors, mechanisms other than loss of ER expression are responsible for treatment failure. We have investigated the occurrence and frequency of the exon-5-deletion variant (d5) of ER in human breast-cancer biopsies and in normal tissues. In all normal and tumor tissues tested, both wild-type (wt) and d5 were detected, indicating that expression of the d5 variant is a naturally occurring polymorphism. However, the primary tumors of patients who relapse within 15 months (n = 13) express higher ratios of d5 than do those of patients with no relapse during the same period (p = 0.4, n = 19), though this difference is statistically not significant. A significant increase in the expression level of d5 was determined in relapse as compared with the respective primary tumor (p = 0.02). These data indicate that increased expression of the ER exon-5-deletion variant in relapse tissues might be due to clonal selection of cells resistant to anti-estrogen treatment.